Natural gas has been used as a fuel for piston engine driven vehicles for over fifty years but the drive to improve efficiency and reduce pollution is causing continual change and improvements in the available technology. In the past, natural gas driven vehicles (NGV) were naturally fumigated, that is, natural gas was introduced into the cylinders through the intake manifold, mixed with the intake air and fed into the cylinders at relatively low pressure. The fuel supply system for such an NGV is relatively simple. Fuel is held in and supplied from a liquefied natural gas (LNG) vehicle tank with working pressure just above the engine inlet pressure, or from compressed natural gas cylinders (CNG) through regulators that reduce the pressure to the engine inlet pressure.
Compressed natural gas (CNG) is commonly stored at ambient temperatures at pressures up to 3600 pounds per square inch (24,925 kPa), and is unsuitable for trucks and buses due to the limited operating range and heavy weight of the CNG storage tanks.
On the other hand, liquefied natural gas (LNG) is normally stored at temperatures of between about −240° F. and −175° F. (about −150° C. and −115° C.) and at pressures of between about 15 and 200 psig (204 and 1477 kPa) in a cryogenic tank, providing an energy density of about four times that of CNG.
However, better efficiency and emissions can be achieved if the natural gas is injected directly into the cylinders under high pressure at the end of the compression stroke of the piston. This requires a fuel supply system that can deliver the natural gas at a pressure of 3000 pounds per square inch gauge (psig) and above. This makes it impossible to deliver the fuel directly from a conventional LNG vehicle tank and it is impractical and uneconomical to build an LNG tank with such a high operating pressure. Equally, it is impossible to deliver the natural gas fuel directly from a conventional CNG tank as the pressure in such a tank is lower than the injection pressure as soon as a small amount of fuel has been withdrawn from the CNG tank. In both cases, a booster pump is required to boost the pressure from storage pressure to injection pressure.
Liquid Natural Gas (LNG) Pump
High pressure cryogenic pumps have been on the market for many years, but it has proven difficult to adapt these pumps to the size and demand of a vehicle pump. In general, cryogenic pumps should have a positive suction pressure. It has therefore been common practice to place the pump directly in the liquid so that the head of the liquid will supply the desired pressure. The problem with this approach is that it introduces a large heat leak into the LNG storage tank and consequently reduces the holding time of the tank. The holding time is the time it takes for the pressure to reach relief valve set pressure.
Some manufacturers have placed the pump outside the storage tank and have reduced the required suction pressure by using a large first stage suction chamber. The excess LNG which is drawn into such a chamber, over that which can fill a second chamber, is returned to the LNG tank and again, additional heat is introduced into the LNG, which is undesirable.
Another problem with a pumped LNG supply is that it is difficult to remove vapor from the LNG storage tank. With low pressure gas supply systems, this is easily done. If the pressure in the LNG tank is high, fuel is supplied from the vapor phase thereby reducing the pressure. If pressure is low, fuel is supplied from the liquid phase. This characteristic of a low pressure system substantially lengthens the holding time, which is very desirable as mentioned above. Extending the holding time cannot be done with conventional LNG pump systems that draw from the liquid phase only and cannot remove vapor.
Gram U.S. Pat. No. 5,411,374, issued May 2, 1995, and its two divisional patents, U.S. Pat. No. 5,477,690, issued Dec. 26, 1995, and U.S. Pat. No. 5,551,488, issued Sep. 3, 1996, disclose embodiments of a cryogenic fluid pump system and method of pumping cryogenic fluid. The cryogenic fluid piston pump functions as a stationary dispensing pump, mobile vehicle fuel pump, and the like, and can pump vapor and liquid efficiently even at negative feed pressures, thus permitting pump location outside a liquid container. The piston inducts fluid by removing vapor from liquid in an inlet conduit faster than the liquid therein can vaporize by absorbing heat and causing pressure in the inlet conduit to be lowered. The differential pressure between the vapor pressure in the conduit and the vapor pressure within the tank pushes the liquid into the pump. The piston moves at essentially constant velocity throughout an induction stroke to generate an essentially steady state induction flow with negligible restriction of flow through an inlet port. The stroke displacement volume is at least two orders of magnitude greater than residual or dead volume remaining in cylinder during stroke changeover, and is greater than the volume of inlet conduit. As a fuel pump, the pump selectively receives cryogenic liquid and vapor from respective conduits communicating with the tank, and pumps cryogenic liquid to satisfy relatively heavy fuel demand of the engine, which, when satisfied, also pumps vapor to reduce vapor pressure in the tank while sometimes satisfying relatively lighter fuel demand.
Conventional, prior art cryogenic pumps are typically centrifugal pumps, which are placed either in the liquid inside the storage tank, or below the storage tank in a separate chamber with a large suction line leading from the tank, with both the pump and suction line being well insulated. Because a cryogenic liquid is at its boiling temperature when stored, heat leaked into the suction line and reduction in pressure will cause vapor to be formed. Thus, if the centrifugal pump is placed outside the tank, vapor is formed and the vapor will cause the pump to cavitate and the flow to stop. Consequently, prior art cryogenic pumps require a positive feed pressure to prevent or reduce the tendency to cavitation of the pump. In a stationary system, the positive feed pressure is typically attained by locating the pump several feet, for example, 5-10 feet (about 2-3 meters) below the lowest level of the liquid within the tank, and such installations are usually very costly. On board fuel storage systems for vehicles use other ways to provide positive feed pressure. Also, centrifugal pumps cannot easily generate high discharge pressures to properly inject fuel directly into the cylinder of an internal combustion engine and that are also desirable to reduce fuelling time for fuelling station applications.
Reciprocating piston pumps have been used for pumping LNG when high discharge pressures are required or desired, but such pumps also require a positive feed pressure to reduce efficiency losses that can arise with a relatively high speed piston pump. Prior art LNG piston pumps are crankshaft driven at between 200 and 500 RPM with relatively small displacements of approximately 10 cubic inches (164 cubic centimeters). Such pumps are commonly used for developing high pressures required for filling CNG cylinders and usually have a relatively low delivery capacity of up to about 5 gallons per minute (20 liters per minute). Such pumps are single acting, that is, they have a single chamber in which an induction stroke is followed by a discharge stroke, and thus the inlet flow will be stopped half of the time while the piston executes the discharge stroke. Furthermore, as the piston is driven by a crank shaft which produces quasi-simple harmonic motion, the piston has a velocity which changes constantly throughout its stroke, with 70% of the displacement of the piston taking place during the time of one-half of the cycle, that is, one-half of the stroke, and 30% of the piston displacement occurring in the remaining half cycle time. The variations in speed of the piston are repeated 200-500 times per minute, and generate corresponding pressure pulses in the inlet conduit, which cause the liquid to vaporize and condense rapidly. This results in zero inlet flow unless gravity or an inlet pressure above boiling pressure of the liquid forces the liquid into the pump. In addition, the relatively small displacement of these pumps results in relatively small inlet valves which, when opened, tend to unduly restrict flow through the valves. Thus, such pumps require a positive inlet or feed pressure of about 5 to 10 psig (135 to 170 kPa) at the feed or inlet of the reciprocating pump unless the inlet valve is submerged in the cryogenic liquid in which case the feed pressure can be reduced. Large cryogenic piston pumps, with a capacity of about 40 gallons per minute (150 liters per minute) have been built, but such pumps are designed for very high pressure delivery, require a positive feed pressure and are extremely costly.